Manufacture of thermionic cathodes



1,952,854 PATENT OFFiCE MANUFACTURE OF THERMIONIC CATHODES August Gehrts, Friedenau, Berlin, Germany, assignor to Siemens and Halske Aktiengesellschaft, Siemensstadt, near Berlin, Germany, a

German Company No Drawing. Application September 9, 1931, Serial No. 561,998. In Germany October 22,

7 Claims.

This invention relates to the manufacture of thermionic cathodes and more particularly to the manufacture of oxide cathodes.

When making oxide cathodes, it is usual to proceed in the following manner. A paste is applied to a carrier wire consisting of a high melting material, the paste containing the highly emissive substance. The oxide cathode manufactured in this way is sealed into a discharge vessel and then subjected to an electro-thermal treatment so that the full emissive power is obtained. It was soon discovered that the use of pure carrier materials, such as tungsten, platinum, molybdenum, etc., did not permit of the manufacture of durable cathodes. Materials, such as nickel, iridium and rhodium were then added to the carrier material. By this addition the cathode became considerably more stable and it was much easier to form and activate the cathode, that is to say, bring it to its full power. The operations taking place during the forming and activating process can be imagined to be somewhat as follows:

If, for instance, barium oxide is used as an oxide substance and an alloy of platinum and nickel as the core material, it is easy to transform the whole barium oxide into a nickelite. The compound decomposes on glowing in a vacuum, giving off a large quantity of oxygen and a finely divided nickel remains in the layer applied. This process is called forming the cathode. If a current is now taken from the cathode, for instance, for degassing the anode and the grid by electron bombardment, the oxygen residue still present in the layer is driven out, barium difiuses on the surface, (the temperature drop between core and surface can be considered in this case as a driving force) and thus an activation takes place on the surface. By suitable means it is possible to so conduct the forming and activating process that the layer only contains barium in metallic form, if necessary, substantially absorbed on to strontium oxide which is mixed with the initial materials. On cathodes previously treated in this way, no oxidation of the core material (iridium, rhodium, nickel, more generally speaking of the oxidizable parts of the core material) can take place; the cathode is chemically stable. Since no further escape of the alloy components of the core material can take place on cathodes of this kind, the filament voltage can also not vary during long service (heating by current presumed). The forming and activating process described above must be very strictly adhered to and is therefore to be considered a dificulty in manufacture.

In accordance with this invention, the method of making formed oxide cathodes in which an oxide layer is situated on a suitable high melting carrier metal, consists in adding colloidal materials, such as nickel, iridium, rhodium and others to the oxide or oxide mixture or rather the compounds which are transformed into oxides on heating, whereby a good formation of the cathode is accomplished.

The method can, for instance, be so employed that the oxide mixture is first made and the finest divided nickel is then added to this mixture in colloidal form. If the mixture made in this way is then applied to the carrier wire, a cathode of this kind can, after mounting and heating, be considered as formed. The forming process can be still further assisted by using an alloy carrier wire so that additional forming takes place from the interior outwardly.

What is claimed is:

1. The method of making oxide coated cathodes which comprises preparing a mixture of colloidal metal and alkaline earth metal compounds, the colloidal metal consisting of a metal of the eighth periodic group, coating a metallic core suitable for a cathode with said mixture, and transforming said mixture.

2. The method of making oxide coated cathodes which comprises preparing a mixture of colloidal metal and alkaline earth metal compounds, the colloidal metal being a metal of the eighth periodic group, coating a metal core suitable for a cathode with said mixture, decomposing said mixture, and activating said cathode in a vacuum.

3. The method of making coated cathodes which comprises adding colloidal nickel to alkaline earth metal compounds, coating a metallic core with said nickel and compounds, decomposing said compounds to form nickelites and alkaline earth oxides, withdrawing oxygen released by said decomposition, and converting said oxides to metal and said nickelites to nickel.

4. The method of making a cathode coated with emissive material which comprises coating an alloy core of platinum and nickel with a mixture of colloidal nickel and alkaline earth compounds, simultaneously decomposing said coating from within and exterior to said core to change said compounds to oxides, and thereafter activating said coating to form a layer of alkaline earth metal on said oxides.

5. The method according to claim 1 in which the colloidal metal is nickel.

6. The method according to claim 1 in which the colloidal metal is rhodium.

7. The method according to claim 1 in which the colloidal metal is iridium.

AUGUST GEHRTS. 

